1. Field of the Invention
The present invention relates to a withstand voltage-testing apparatus and more particularly to the withstand voltage-testing apparatus preferably used to measure the withstand voltage of a high-voltage component part such as a high-voltage cable or the like connected with an ignition plug of an internal combustion engine. In the apparatus, in order to detect whether electric current has leaked from the high-voltage component part, a current transformer and a resistor are used to convert the leakage current into a voltage, if the leakage current has been generated therein.
2. Description of the Prior Art
The following first and second methods are adopted to measure the withstand voltage of a work such as a high voltage cable connected with an ignition plug of an automobile.
In the first method, an automatic deciding testing apparatus of discharge type shown in FIG. 3 is used. In the second method, an operator checks whether electric current has leaked from the work by using a ground bar, as shown in FIG. 4.
In the automatic deciding testing apparatus of discharge type shown in FIG. 3, a core conductor 2 of a high-voltage cable 1 used as a work is covered with an insulating material 3, and the insulating material 3 is surrounded with a ground conductor 4. As a deciding device, a triple-needle discharge device 5 comprising a positive pole 5a, a negative pole 5b, and a floating pole 5c is used. The positive pole 5a is connected with the core conductor 2, while the negative pole 5b is connected with the ground conductor 4. A high-voltage pulse is applied to the core conductor 2 and the positive pole 5a of the discharge device 5 by a high-voltage pulse generator 6 comprising a high-voltage generation circuit 6a and an oscillation circuit 6b.
In order to decide whether or not electric current has leaked from the high-voltage cable 1 by means of discharge light generated in the gap in the three-pole needle, the apparatus has an optical leakage current-detecting means comprising a light receiving section 8 for receiving the discharge light and a deciding section 9. The light receiving section 8 comprises an optical fiber 8a and a high-speed photodiode 8b which generates an inverse voltage, upon receipt of light introduced to the light receiving section 8 via the optical fiber 8a. The deciding section 9 comprises an OSC synchronization comparator 9a which compares with each other a high-voltage pulse (b) applied to the core conductor 2 and the positive pole 5a of the discharge device 5 and the pulse (a) of discharge light applied by the light receiving section 8 so as to check whether both pulses are synchronized with each other; and a deciding circuit 9b for deciding whether the high-voltage cable 1 is defective or undefective, upon receipt of a signal, from the OSC synchronization comparator 9a, indicating that the two pulses have been synchronized with each other or they have not been synchronized with each other.
If no electric current has not leaked from the high-voltage cable 1 in the test, it is decided that the high-voltage pulse (b) and the pulse (a) of the discharge light have been synchronized with each other and thus the high-voltage cable 1 is "undefective" If electric current has leaked from the high-voltage cable 1 because the high-voltage cable 1 has a pin hole or the like thereon, discharge does not take place in the discharge device 5 or unstable discharge light is generated. Consequently, the pulse (a) outputted from the photodiode 8b is not synchronized with the pulse (b). That is, it is decided that the high-voltage cable 1 is "defective".
In the second method shown in FIG. 4, the operator applies a high voltage to the work 1 (high-voltage cable) via a ground bar 10 by applying the latter to the former so as to check visually whether electric current has leaked from the work 1 or not. That is, if the work 1 has a pin hole P thereon, leakage light is generated between the ground bar 10 and the work 1, whereas if the work 1 has no pin hole P thereon and thus if no electric current leaks from the work 1, leakage light is not generated therebetween.
The first testing method, namely, the method to be carried out by using the automatic deciding apparatus of discharge type has the following problems because the three-pole needle is used as the discharge device 5.
(1) It is necessary to change the voltage of the discharge device in correspondence to the change in the voltage to be applied to the work 1. To this end, it is necessary to adjust the length of the gap among the positive pole 5a, the negative pole 5b, and the floating pole 5c of the three-pole needle by changing the positions thereof. Accordingly, it takes much time and labor to conduct tests and further, it is difficult for the apparatus to measure the withstand voltage of the work 1 accurately. PA1 (2) When a high voltage is applied to the work 1, it is necessary to provide a long gap G in the discharge device 5. In this case, unstable discharge light is generated in the gap G. As a result, although the discharge light is continuously generated, there is a possibility that discharge light having regular pulses cannot be generated. In this case, it is decided that the work 1 is insulated defectively. PA1 (3) Because discharge is performed in atmosphere by using the discharge device comprising the three-pole needle, a discharge voltage changes by .+-.SKV and is thus unstable. Consequently, evaluated results are not highly reliable. PA1 (4) Because the discharge is carried out in atmosphere, ozone and a great sound are generated, and hence an unfavorable environment is produced in the periphery of a place in which tests are conducted.
Accordingly, the upper limit of the voltage to be applied to the work 1 is 25-28 KV. At a voltage higher than 28 KV, a withstand voltage-checking test cannot be conducted by using the discharge device comprising the three-pole needle.
In the second method, whether the work 1 is defective or not is decided visually. Thus, a test result depends on operator's professional skill and experience. Thus, test results are not highly reliable and hence, there is a possibility that a defective work is decided as an undefective work.